The present invention generally relates to rotational molding, and, more particularly, to a method and apparatus for forming superior molded parts using a rotational mold having a moving insert.
Rotational molding, also known as rotomolding, is a production process in which a liquid or powdered thermoplastic is charged into a hollow mold and then rotated continuously in a biaxial mode in a high temperature environment to form hollow complex parts. Rotational molding has become widely adopted, particularly for forming complex, hollow parts, many of which cannot be manufactured by any other commercially feasible process. The mold surface is reproduced exactly, and there is little or no requirement for finishing work.
It is apparent that the distribution of molding material within the mold is carried out only by the force of gravity applied to the liquid thermoplastic material as the mold is rotated. Generally this technique is very effective in replicating the minute details of the mold surface, and to deposit a generally uniform sidewall buildup on the mold surface. As a result, rotational molding has been found to produce reliable, uniform parts in a great variety of shapes and sizes.
Rotational molding is not without drawbacks, however. It is known in the art that component configurations that require opposed sides of the mold to be in close proximity may cause poor quality parts to be molded. The liquid thermoplastic material has viscosity and surface tension properties that may limit or inhibit the flow of the liquid material between opposed, closely spaced mold surfaces. The limited material flow can lead to thin wall portions or voids in the sidewall that may compromise the structural integrity of the molded part, create a path for leaks into or out of the molded part, or the like. Thus some desirable structural features commonly used to impart strength or structural rigidity, such as a solid rib formed at the peripheral edge of the sidewall of the molded part, cannot be molded reliably, nor can integral structural coffers be formed in the sidewall if they are too closely spaced or touching the opposite sidewall. These limitations may constrain the design freedom of a designer of molded parts.
The present invention generally comprises a method and apparatus for rotational molding that enables the production of a wider range of molded parts. In particular, the invention provides a molding technique that produces features heretofore unattainable, such as a solid rib formed at the peripheral edge of the sidewall of the molded part, or closely spaced opposed surfaces.
In one aspect, the invention provides a rotational mold having first and second mold components defining therebetween a mold cavity. The first mold component is provided with a mold insert that is adapted to be translated toward the second mold component to an inward position in the mold cavity. The first mold component includes spring assemblies for resiliently biasing the mold insert inwardly, and a latch mechanism is also provided to restrain the mold insert in the outward disposition. The latch mechanism includes a touch pad which may be struck to trigger the latch mechanism and permit the mold insert to be driven by the spring assemblies inwardly into the mold cavity.
In another aspect, the invention includes a method for rotational molding that employs the rotational mold described above. With the mold insert latched in the outward disposition, the rotational mold is separated and charged with thermoplastic material, closed and sealed, and placed in a furnace and rotated biaxially. After the heating phase is finished, the rotating mold is moved to a cooling station. During the cooling process and while the mold assembly continues to rotate biaxially, the touch pad of the latch is struck to release the latch and cause the mold insert to move inwardly. The mold insert includes structural surfaces and features that are moved more proximate to opposed mold portions. As a result, component features that are too closely spaced to be reliably molded by prior art rotational techniques are fully formed by the mold insert in a more spaced apart relationship and brought into close spacing or impingement by movement of the mold insert. Thus closely spaced structures that were heretofore unattainable by rotational molding may be created simply and reliably.
The method and apparatus described above may be used to fabricate a bus bumper cover by rotational molding techniques. The bumper cover includes a longitudinally extending housing having front and rear sidewalls in spaced apart relationship, with a tail panel extending obliquely from one end of the housing. The peripheral edge of the housing includes a thick, solid rib extending along the peripheral edge thereof. Also, a plurality of concave coffers extend inwardly in the inner sidewall to join the front sidewall for enhanced structural strength and impact resistance. The solid rib portions and the concave structural coffers are formed by the molding surface of the mold insert described above, and are properly and fully formed during rotational heating of the mold with the mold insert is extended in the outward position. After the heating stage is completed and while the mold continues to be rotated biaxially, the latch is triggered to move the mold insert inwardly, so that the fully formed surface features within the mold may be brought into a closely spaced or impinging relationship that would otherwise be impossible to obtain with prior art rotational molding techniques.